Tips

Clicking the label for any input or result will cause a
popup help box to appear. This help box includes the
allowed and normal values (for inputs).
Read more.

This is a "solve-all" tool. Select the radio button to the left of the
label for which you wish to solve. This label will turn to
blue and the input box will disappear.

Some instantaneous (demand) water heaters have a fixed output, which
increased the water temperature by a fixed number of degrees for a
given flow rate. Others have a variable output (firing rate) that
increases as the flow rate increases. It is important to know the
characteristics of the appliance before sizing.

Inputs and field measurements

Recovery efficiency - the rated recovery efficiency of the water heater.
Enter as a percent; 78 instead of 0.78. For most water heaters, standby
losses are minimal so that the recovery efficiency is essentially the
same as the energy factor (EF). A case in which these efficiency values
are substantially different would be a tankless coil in a boiler, where
standby losses in the summer are a large fraction of the total energy
input. In that case, the steady-state efficiency (SSE) is likely to be
a good approximation of the recovery efficiency.

Because this is a "solve-all" tool, each of the values below will be
treated as inputs unless selected as your result using the
radio button to the left of the label.

Temperature rise - the difference between the temperature of the cold
water entering the water heater and the heated water leaving the appliance.

Peak flow rate - the design hot water flow rate. See Table 1, below,
for flow rates for mixed (hot + cold) water uses. Use the
DHW Volume per Use tool to determine the hot-water-use portion of
this flow.

Best practices

Select a water heater that has an input heat rate that is equal to
or greater than the "Input heat rate" tool result.

References

ASHRAE. 2011 HVAC Applications Handbook, p. 50-25. Atlanta, GA: American Society
of Heating, Refrigerating and Air-Conditioning Engineers, Inc. This document
may be purchased at
www.ashrae.org.
Comment: This document includes a chapter on service
water heating and specifically sizing tankless [instantaneous] water
heaters - Chapter 50. The information in Table 1 is from this book.

Related tools

DHW Systems Comparison:
This tool allows you to calculate the annual cost of heating domestic
hot water. In addition, you may compare the annual costs of up to
three systems to a base system. Four water heating fuel types are
available; electricity, natural gas, LPG, and #2 oil. You can use this
tool for any system for which you have an energy factor (EF), including
electric resistance and heat pump systems, and various types of
oil- and gas-fired systems.

DHW Average Daily Usage:
This tool calculates average daily hot water use based on the amount of
hot water used by various appliances and use incidents. Default usage values
are included.

DHW First Hour Rating:
This tool helps you determine the First Hour Rating for the purpose of
sizing a storage water heater. This method is based on guidance from the
Air-Conditioning, Heating and Refrigeration Institute (AHRI), the
industry standard for sizing storage water heaters.

DHW Volume per Use:
This tool determines the volume of hot water used given an amount of mixed
(hot + cold) water used and the temperatures of the hot, cold, and mixed water.
The amount of mixed water can be specified by volume directly, or from a flow
rate and duration.

Water Flow Rate:
This is a "solve-all" tool that relates the volume of water drawn, the
duration of the water draw, and the flow rate. The primary expected use
case is determining flow rate using a test container with known volume
and a stop watch.

Related external resources

American Council for an Energy-Efficient Economy (ACEEE).
Water Heating:
http://aceee.org/consumer/water-heating#storage
Comment: This web page includes updated information
about water heater types and sizing, life-cycle cost comparisons, sizing a new
system, and efficiency standards.

California Energy Commission, Consumer Energy Center. Water Heaters:
www.consumerenergycenter.org/home/appliances/waterheaters.html
Comment: This web page includes a very good overview
of water heater issues, including pros and cons of the different types,
purchasing the right water heater, understanding energy factor, and
saving water heater energy.